Process control for hypochlorite production



Aug. 10, l965 F. cLERBols ETAL PROCESS CONTROL FOR HYPOCHLORITE PRODUCTION Filed July 10, 1962 4 Sheets-Sheet 1 FRANCO/S CLERO/S ,00L ANTOINE Bymbm AwoRNEYal AU8- 10 1955 cLERBols ETAL 3,199,949

PROCESS CONTROL FOR HYPOCHLORITE PRODUCTION Filed July 10, 1962 4 Sheets-Sheet 2 *Px* f Cao/er t f E 9| R U @j 1Q @I e 'E Gx Q .Q M t ol k g N i' s I Q I ek Q. N. l 1 sz Eb v E- N u:

INVENTORS FRANCO/5 CLL-'R606 ,00L ANNO/NE ATroRNEYi` Aug- 10, 1965 F. cLERBols ETAL PROCESS CONTROL FOR HYPOCHLORITE PRODUCTION 4 Sheets-Sheet 3 Filed July l0, 1962 ATTORNEYS Aug. 10, 1965 F. cLERBols ETAL PROCESS CONTROL FOR HYPOCHLORITE PRODUCTION Filed July 10, 1962 4 Sheets-Sheet 4 IIAING INVENTORS FRANCO/5 CLER/S POL A/Vm//VE ATTORNEYS United States Patent O 3,199,949 PROCESS CONTROL FOR HYPOCHLORITE PRODUCTION Francois `Clerbois, Uccle-Brussels, and Pol Antoine,

Jemeppe-sur-Sambre, Belgium, assiguors to Solvay &

Cie, Brussels, Belgium Filed July 10, 1962, Ser. No. 208,817 Claims. (Cl. 23-86) This invention relates to -a process for producing hypochlorite by a reaction lbetween gaseous -chlorine and an alkaline solution, the reactants being introduced into a reactor in a ratio which is adjusted to avoid decomposition of the formed hypochlorite.

It is known that chlorine in alkaline solutions, particularly in solutions of caustic soda, is converted to hypochlorite according to the equation:

This reaction requires the presence of excess base, for in the absence thereof, the formed hypochlorite is decomposed either pa-r-tially or totally. Consequently, insulicient alkalinity results in a -loss of product, and may even lead to an explosive decomposition. Hence, the decomposition of the hypochlorite must be avoided from both the economic and safety standpoints.

This decomposition can, in fact, be avoided by regulating the rate of ilow of the chlorine in two periods during the chlorination: at the beginning of the operation, a large quan-tity of chlorine is introduced into the concentrated alkaline solution; then, when the yreaction is suf'liciently advanced, the flow of chlorine is greatly reduced and the reaction is completed at a reduced rate.

As another mode of operation, automatically controlled valves have been used to regulate the chlorine or caustic soda feed stream, said valves being adjusted in prompt response to the redox potential yof the hypochlorite solution. Based on this variable, suitable apparatus for the continuous production of sodium hypochlorite provides for the passage of an alkaline solution of a controlled quantity through a reactor into which chlorine is injected. The formed hypochlorite solution is passed over electrodes which are used to measure the oxidation-reduction potential, said measurement reiiecting the quantity 4of the alkali present in the solution; and the value of the redox potent tial in turn, serves to regulate a valve controlling the flow of the chlorine injected into the reactor. Alternatively,

it is possible to regulate the introduction of the sodiumv hydroxide automatically in place of, or in addition to, the chlorine feed.

It is important to note, however, that the control of the `supply of one or the other of the reagents to the chlorination reactor in response to the oxidation-reduction potential of the solution discharged therefrom entails serious drawbacks. For example, the redox potential depends to a large degree on the type of the utilized electrodes, their position and orientation. Still further, a much more significant disadvantage is based on the fact that the measurements of the redox potential are far Vfrom being reproducible in the hypochlorite solutions;

and this lack of reproducibility is, unfortunately, inherent in this redox medium. Accordingly, this inconsistency of the measurements makes it very difficult, if not hazardous, to control the reaction for the production of hy-pochlorite.

"ice

The principal object 0f this invention is to provide` a controlled system which is safe, effective and reliable for the substantial prevention of hypochlorite decomposition in a process of producing hypochlorite by reacting chlorine with an alkaline solution.

Stil-l other objects and advantages of this invention will become apparent upon further study of the specifi-cation and appended claims.

According to this invention, the decomposition of the hypochlorite produced in a reaction between gaseous chlorine and an alkaline solution introduced into a reactor in variable proportions can be avoided by continuously so adjusting the proportions of the chlorine and alkaline solution that the derivative with respect -to time of the oxidation-reduction potential measured in the liquid contained in the reactor or withdrawn from it, is not subjected to any considerable variation.

Referring to the attached drawings, FIGURE l is a graph showing the relationship of the first derivative of the redox (oxidation-reduction) potential vs. time; FIG- URES 2, 3, and 4 show different arrangements of apparatus which can be employed to utilize the process of the invention.

Indeed, it has been found that although the measured Values of the oxidation-reduction potential in the hypochlorite solutions vary considerably, the derivative of this potential with respect to time lalways remains within reproducible values, and precisely characterizes Ithe progress of the chlorination reaction.

The oxidation-reduction potential in a hyp-ochlori-te solution of soda is based on the following reaction:

assuming that the decomposition reaction of the hypochlorite is negligible.

The following equation results if R1 is the oxidationreduction potential in a hypochlorite solution:

R0 is the standard potential R is the perfect gas constant T is the absolute temperature K.

F is the Faraday constant nis the number of exchanged electrons Since (ClO)=(Cl-), the derivative of R1 with respect 'to time is:

@Q -ZRT 1 d(OH-) di F [0H-1 d:

inwhich wir It follows therefrom that a system based on the variations of the derivative presents the following advantages for the regu-lation of the proportion of the reagentsg and in particular of chlorine:

(l) When there is a large excess of base, the rate of change of the concentration of base does not affect the derivative of the redox potential significantly. Thus, when the possibility of hypochl'orite decomposition is nil due to a strong concentration of base, the capacity (inertia) of the controlled system is desirably high.

These reactions are accompanied by a very marked and considerable increase of the derivative of the redox potential in proportion to time, as can be seen from FIG- URE 1. In fact, this ligure shows that in the beginningof the reaction the derivative varies slightly, for example, that it 'increases slowly from 0 to 3 m./min., as long as the chlorination of the alkaline solution occurs normally Without decomposition of the hypochlorite. However, when the alkalinity reserve `of the hypochlorite solution is exhausted, the derivative of the redox potential increases suddenly and then continues very krapidly ,ad

infinitum, because at this moment the variation of the redox potential as a function of time is discontinuous,k

The process is controlled by regulating the proportions f reagents so that the derivative of the redox potential with respect to the time does not exceed a predetermined value which is higher than the value of dRl/dt which/,is reached during the -iirst stage of the process, i.e., when only the reaction for the formation of hypochlorite is taking place as shown in FIG. 1 for 0 to 38 minutes. The

Vmaximum value of the derivative is determined by taking into account the characteristics ofthe installation of the automatic control system. In general, this value is fixed at a limit of from 20 to 50 times the value of the derivative attained in the rst stage of the process and falls on the curve of FIG. l beyond 38 minutes. kHowever, since the derivative varies very rapidly as a function of time A according'to anasymptotic curve, it is possible to set the maximum value of the derivative at a value which isdistinctly higher since the automatic apparatus responds very rapidly. e i

It is also important to note that even if the variation of the derivative of the redox potential, dRi'/ dt, as a function of time, is characteristic of the reactions occurring during the two phases of the process and independent of the operating conditions, the absolute value of this derivative is nevertheless dependent on said operating conditions.

The process according tothe invention can be ladvantageously applied to all the industrial methods for manufacturing hypochlorite. .In all casesrthere .is obtained a lessening of the possibility of hypochlorite decomposition, and moreover, a higher quality of the manufactured product as well as an improvement inthe yield.

The` process fof this invention is particularly advantageous where the controlof the reagent proportions is very dicult. For example, when a batch chlorination of a certain quantity of alkaline solution is conducted by the injection of vaporized, chlorine into a suitable reactor,

it is always of interest to increase the productivity of the .equipment andl to reduce the durationof the process by voperating with: the highest possible chlorine proportion. VThis cannot be done according to known processes'because ofthe hazards involved with'hypochlorite decomposition. As compared therewith, when the lchlorine proportion is automatically regulated according to this invention in such a way that the derivative of the redox potential of the hypochlorite solution varies not significantly during the course of the process, the maximum chlorine proportions can be employed at all times, thereby effect- Y i ing the chlorination within aminimum of time while simultaneously avoiding the decomposition of hypochlorite.

The invention can also be advantageously applied to a 4continuous manufacturing process of hypochlorite, for instance, by adjusting the proportion of the alkaline feed Solution to the chlorinating apparatus in response to the Variation of the derivative of the redox potential of the solution discharged from said apparatus.

A particularly economically worthwhile application of this invention linvolves the utilization of residual chlorine from an electrolytic chlorine-producing cell, which resifual chlorine cannot be lconsumed in other manufacturing processes. It is obvious that ,sucha process must be operable with highly variable chlorine proporti'ons'and concentrations and that it is indispensible that substantially all the chlorine be transformed into `hypochlorite, lest it be lost. The residual chlorine raw material is characterized by a most irregular flow and chlorine content. The residual chlorine is generally collected all over the chlorine utilizing plants and particularly at the'chlorine liquefaction plant.

For the utilization of residual chlorine in the process of this invention, variousarrangernents of equipment may .beemployed A preferred installation can comprise two separate and identical` chlorinating apparatuses. One, being in the chlorination stage, receives the available chlo- Y rine, and the other one, being filled with fresh alkaline solution, is kept in reserve and may receive any excess chlorine which cannot be'directed into the first reactor,

v lest the hypochlorite ybe. decomposed. According to this invention, the distribution of the chlorine between the two reactors is adjusted automatically in such a way that there l solution ofthe rst chlorinating apparatus.

In an arrangementof another type functioning according to this invention, the proportion and/or the concentrationV ofthe .alkalinel solution-.is continuously regulated by one or several valves which are automatically controlled in response tothe variations of the derivative with respect to time of the Vredox -potenti'alrmeasured in the reactor liquid or an Youtlet tap, so that the Vquantity of alkali introduced into the reactor increases whenthe de- Vrivativeof the redox potential increasesv abnormally.

Still another `arrangement of apparatus for the production of hypochlorite from residual chlorine in variable quantities and concentrations, comprisesybut one chlorinating apparatus. An installation of this type comprises a soleI chlorinator which receives all ofthe available chlorine at every moment; a tank which contains a measured V'quantity of partially chlorinated liquid; and injection devices for chlorine .and the fresh, alkaline solution. In such an installation functioning, for'instance, continuously, the valve which regulates the admission of fresh alkaline solution is controlled in response to the 'derivativeof theredox potential thereby resulting in the `ing apparatus, the -production costs of the hypochlorite ,canbe considerably. reduced, and concomitantiy, the yield per unit ofequipment is increasedV without any supple- `mentaryfexpense. Y l

However, when the .variations inthe proportion `or the concentration of the chlorine are considerable, the volume of the solution containedv in the installation can exceed the capacity of equipment. In certain extreme cases, an almost complete emptying of the installation has been observed as a consequence of the tapping of an excessive quantity of produced hypochlorite or, on the contrary, an overow of the liquid in case too much fresh alkaline solution has been introduced.

Such things can happen when the chlorination rate of the total solution is not within certain limits. This rate is definedy as being equal to the quantity of alkaline solution reacting per unit of time in the installation. If this rate is too low, frequent additions of fresh alkaline solutions canl cause the volume of the solution to exceed the capacity of the installation thus causing an overflow. It, on the other hand, said rate is too high, the automatic discharging device which operates as soon as the hypochlorite concentration attains the desired value, can discharge too much solution. Such drawbacks, which arise only under very particular conditions, can be overcome by improving the automatic regulation in such a way that the chlorination rate of the total solution is taken into account.

In order to achieve this improved automatic control, the quantity of the fresh alkaline solution introduced into the reactor is continuously regulated by one or several automatically controlled valves, and simultaneously by the variations of the derivative with respect to time of the redox potential measured in the liquid contained in, or withdrawn from, the reactor, and the rate at which the chlorination of the total solution proceeds. In this way, the quantity of alkali introduced into the reactor increases when the derivative of the redox potential increases abnormally, and that as said quantity becomes greater, the

higher becomes the rate of chlorination of the total solution.

Said rate can be ,measuredv byyany adequate means,

for instance, by measuring the speed at which the chlorine is introduced into the chlorinating apparatus, since the chlorination reaction, as such, occurs almost instantaneously. lt is preferred, however, to measure the chlorination rate of the total solution by the derivative of the redox potential measured in the liquid in the tank containing the measured quantity of the solution, or at the entrance of the chlorinating apparatus, dRZ/dt as shown at 4 in FIG. 4.

Said derivative, or any. other parameter by which the chlorinating speed of the total solution is measured in the improved installation, acts in conjunction with the derivative of the redox potential measured in the reactor liquid or outlet, dRl/dt as shown at 4 in FIG. 4, to control the valve regulating the admission of the alkaline lye. These two data can be combined for eiiecting the control of the valve either by a mechanical or pneumatical coupling of the two regulators controlled by the derivatives, or by a combined electronic device, for instance in a computer.

Such a combination can be realized in a way that for each value of the chlorination speed of the total solution,

or for each value of the parameter by which it is measured, as for instance dRz/dt as shown at 4 in FlG. 4,

there corresponds a particular position of the regulating device for the admission valve, of the alkali.V That is to say, that the values lof the derivative dRl/dt determined at 4 in FIG. 4 for which. the valve is either completelyy opened or closed are, in turn, dependent on the chlorination speed of the total solution. For example, when thevrate of chlorination increases the valve is opened at lower values than ordinarily necessary for the derivative dRl/dt.

It is believed that the preceding description of this invention is sutiiciently complete, clear and concise as to enable one skilled in the art to employ it to thefullest extent. Thus, the following preferred specific embodiments of this invention are merely exemplary, andare not intended to be limitative in any way whatsoever of the broad aspects of the invention as described in the remainder of the specification and appended claims.

Example l1 The production of sodium hypochlorite in a batch process from residual chlorine is advantageously conducted in two separate installations, one of which being always in the effective chlorination stage, the other one containing more concentrated caustic soda lye, being kept in reserve or receiving the excess of chlorine which cannot be supplied to the iirst one.

One of the two installations is schematically illustrated in FIGURE 2. It essentially comprises a tank 1, a circulation pump 2, a chlorinator of the scrubber type 3, a measuring cell Iof the redox potential 4, a cooler 5, a differentiator 6, an electro-pneumatic transmitter 7 anda motor 8, acting on a valve 9 placed in the supply line for the chlorine.

ln the beginning of the operation the tank contains alkaline liquid consisting of caustic :soda or of caustic soda partially chlorinated in a previous process. This solution is circulated through the chlorinator by means of a pump at a continuous llow of 50 m.3/h. (m.3/h.=cubic meters per hour).

The residual chlorine derived lfrom the electrolysis is introduced through conduit l@ and the valve 9 comprising an automatic closing device. At the outlet of the chlorinator, at least one portion of the liquid passes through the cell measuring the redox potential, whereafter the liquid is returned into the tank via the'cooler 5.

The redox potential is measured in a cell consisting of a shiny platinum wire and a calomel (mercurous chloride) electrode saturated with KCl. This potential is then differentiated and the resulting signal actuates the electropneumatic transmitter which in turn actuates a pneumatic motor which, drives the chlorine valve.

In this installation, when the capacity of the .scrubber and the quantities of chlorine and soda are taken into account, it can be seen that at the beginning of the reaction the derivative of the redox potential with respect to time varies little and does not exceed 2 mv./ min. When the proportion of chlorine introduced into the chlorinator is too high, the derivative increases very rapidly. This tendency is counteracted by the chlorine supply valve.

The degree of closure of said valve increases according to the :signal delivered by the ditierentiator, whereby the valve is controlled in response to deviations of at least 10% in a way that it is completely closed at a value of the derivative yof the redox potential which in this case is equal to 40 mv./min.

The following table shows for each of the experiments the final concentrations in chloride ions (Cl-)1 originating `from the decomposition of the hypoclrlorite,i.e., the difference between the total concentration in chloride ions (Cl-)2 and the concentrations of the chloride ions which are normally formed during the course of the chlorination reaction of the caustic soda (Cl-)3. This latter concentration is equal to that of the hypochlorite ions (ClO-)- total chloride ion concentrationziinal chloride ion concentration-l-concentration formed during chlorination:

The advantages ot the invention will become more apparent in reference to the following gures setting forth the highly substantial reduction in thedecomposition of the sodium hypochlorite obtained by control ybasedon the derivative of the redox potential.

All of the experiments were conducted in the aforedescribed apparatus. In a iirst series of experiments, the tlow of chlorine was regulated according to known processes. Another series of experiments was conducted thereafter in the same apparatus, however by regulating the ilow of the chlorine by a pneumatic valve controlled by a diiierentiating device, as described above.

now

hence Cl- 1 mol, g./l. from decomposition of hypochlorite Regulation aco. Regulation acc.

to known to this invention processes It is obvious from the above table that the present process provides the new and unexpected results of decreased hypochlorite decomposition.

Example 2 y FTGURE 3 illustrates another installation for the production of sodium hypochlorite from solutions of caustic sod-a and residual chlorine. This installation comprises a tank 1 provided with a tapping device 12, a circulationV pump 2, a chlorinator of the scrubber type 3, a measuring cell of the redox potential 4, a cooler 5, a differentiating device 6, an electropneumatic transmitter 7, and a motor 8 acting on a valve 9 positioned on the conduit supplying the fresh caustic soda solution from tank 11.

The pump 2 ensures the circulation of the alkaline liquid in the sciubber. This liquid consists of caustic soda which is more or less chlorinated and circulates at a continuous iiow of, for example, 50 m.3/h.

All of the residual chlorine originating from the electrolysis is introduced into the scrubber. At the output` ofthe same, `at least one portion of the chlorinated liquid passes through a measuring cell for the redox potential, whereafter the liquid is reconducted into the tank 1 after having passed through the cooler.

The redox potential is measured in a cell consisting of a brilliant platinum wire and a calomel electrode saturated with KCl. This potential is then differentiated and the resulting signal actuates the electro-pneumatic transmitter which, in turn, acts on a valve that is actuated by a motor. This valve is positioned in a conduit through which the fresh soda solution is conducted from the tank to the scrubber.

It can be seen in this installation, when the capacity of the scrubber and the ows of the chlorine and the soda are taken into account, that under normal conditions, when the alkali is present in excess, the derivative of the redox potential with respect to time changes little and does not exceed 2 ymv./min. When the quantity of chlorine introduced into the chlorinator becomes -too high, that is, when the quantity of alkali present in the solution is in- `suiiicient, the derivative of the redox potential increases very rapidly. At this moment the automatic con-trol device 4acts on the valve positioned on the supply conduit for the caustic soda. The opening of this valve allows for the admission of caustic soda neutralizing t-he excessive chlorine. As soon as the derivative reaches its normal value, the valve closes lagain and the introduction of freshV caustic soda is discontinued.

Thus, `automatic regulation lallows for the production of hypochlorite under normal conditions without decomposition of the formed hypochlorite in spi-te of considerable variations in the How and the concentration of the chlorine introduced into the chlorinator. The produced hypochlorite is Withdrawn from tank 1 by device 12 as soon asrits concentration reaches the desired value.

Example 3 FIGURE 4 illus-trates an installation for the production of'sodium hypochlorite from solutions of caustic soda and residual 4chlorine according toa known process and pro-` vided with an improved regulating system.

This installation comprises a tank 1 provided with a tapping device 12, a circulation pump 2, a chlorinator of Vthe process,

vtl'ie'control and not the rate at which it attains the scrubber type 3, two measuring cells of the redox potential 4, and 4', `a cooler 5, two differentiating devices 6 and 6', an analog computer 13, an electro-pneumatic transmitter 14, and a motor 8`acting on the valve 9 located on the conduit supplying'thefresh caustic soda .solution out oftank 11. *A Y The pump 2 insures the circulation ofthe liquid from tank 1 into the scrubber 3. This liquid consists of partially chlorinated caustic soda, circulates at a constant dow of, for example, 5() m.3/h., and passes at leastin part `through the measuring cell fl for the redox potential.

All the residual chlorine lfrom the electrolysis is intr-od'uced into the scrubber. At the output of the latter, Vat least one portion of .the chlorinated liquid passes through the measuring cell of the oxido-.reduction potential fi, whereafter the liquid is reintroduced into tank 1 after having passed` through the cooler.

The redox potentials are measured at theV output of the scrubber and of thetank by cells consisting of a shining platinum wire and of a calomel electrodesaturated with KCl. These potentials are then differentiating, afterwards combined and the resulting signal actuates-the electropneumatic transmitter which acts in its turn on the valve positioned in the supply conduit of the caustic soda. The opening of the valve increases according to the signal delivered lby the computer. It .is terminated in the considered case when dRl/cz'tat 4 reaches 20 rnv/m. The values of dRg/dt fat 4 ensure the opening of the valve for values of dRl/dt lower than 2() mv./inin. in the case of a rapid chlorination and ensure the closing ofthe valve `at low values of dRl/dt .in case of a .slow chlorination. The values of dR2/dt are setbetween l and 2Q inv/min. according to the desired level in tank 1 and the particular characteristics of the installation. y

The produced hypochlorite is withdrawn from tank 1 by the device 12 as soon as its concentration reaches the desired value.

With respect to the present invention, it is important to lappreciate that the utilization of a control system based on the derivative of the redox potential of the hypochlorite reaction solution is entirely different from the mere selection of .a conventional derivative mode of control in which there is a continuous linear relation between the derivative -function of a controlled variable and the rate at which a final control element attains its equilibrium position. The` latter mode of control is generally, `if not always, used in conjunction with the proportional (linear) mode and reset (integral) mode. In contrast, in the present invention, the derivative function itself is the sole controlling variable. 1 This is, of course, the essence of the in- The differentiation ofthe cell measurement of the redox potential can be accomplished by a differentiating device of vthe type which is schematized in Machine a Calculer Electroniques Arit-hnitiques et nalogiques, M. Plegrin, Dunod, 1959, p. 68.k The output of this diflferentiator actuates a conventional electro-pneumatic transmitter of the type usually employed yin a regulating system.

From the foregoing description, one skilled in the art can easily ascertain the. essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

' For example, it is apparent that this invention, while being directed to the manufacture of hypochlorite, is also amenable to the control of any process wherein the derivative function of a measured variable such as a redox potential, normally remains constant during a reaction, but changes in value suddenly in response to improper oper-ating con- 1In conventional systems, vthe position of `a nal control element 1s fixed by'an independent variableY (ai). In the improved systemhof the present invention, the controlling variable 1s the derivative (dx/dt) of the independent variable of and this derivative determines the position of its position. The derivative function (dm/dt) is thus the independent variable of the regulatlon system of this invention.

ditions. Consequently, such changes and modications are properly, equitably and intended to be Within the full range of equivalence of the following claims.

What is claimed is:

1. In a process for the production of sodium hypochlorite from a solution of caustic soda and a current of gaseous chlorine in a chlorinator having inlet and discharge conduits wherein the concentrations and amounts of the mixture of said sodium hypochlorite, caustic soda and gaseous chlorine vary with time and said amounts of caustic soda and gaseous chlorine are regulated to maintain a slight alkalinity to prevent decomposition of said sodium hypochlorite, said regulation being performed by measurements of the redox potential of said mixture, the improvement comprising measuring said redox potential in at least one of said chlorinator and said inlet and discharge conduits, converting said redox potential into an impulse corresponding to the rst derivative of said redox potential relative to time and regulating by said impulse the addition of at least one of said caustic soda and chlorine, said addition being made in the direction of maintaining substantially constant said rst derivative of said redox potential.

2. The process of claim 1, wherein said chlorine advention. dition is automatically adjusted in response to a deviatio of at least about in the value of said tirst derivative.

3. The process of claim 1, wherein said sodium hydroxide addition is automatically adjusted in response to a deviation of at least about 10% in the value of said rst derivative.

4. The process of claim 1, wherein said sodium hydroxide concentration is automatically adjusted in response to a deviation of at least about 10% in the Value of said first derivative.

5. The process of claim 1, further comprising automatically adjusting at least one feed stream in response to the rate of chlorination of the total reactant solution.

6. The process of claim 5, wherein the addition of sodium hydroxide is automatically increased in response to said rate of chlorination of said reactant solution '7. The process of claim 5, wherein the addition of sodium hydroxide feed is automatically adjusted in response to both said rst derivative and said rate. f

$5. The process of claim 1, wherein said chlorine is obtained from a source which is subject to wide fluctuations in rate, and wherein said addition of chlorine is adjusted in response to abnormal deviations of said rst derivative, and further comprising the step of passing any excess chlorine resulting from said uctuating source and said adjustment to a standby chlorinator filled with sodium hydroxide solution, thereby utilizing the chlorine source to its fullest extent.

9. The process of claim 1, wherein said chlorine is obtained from a source which is subject to wide uctuations in rate, and wherein any excess chlorine fed into said reaction solution is immediately neutralized by automatically increasing the addition of said sodium hydroxide feed stream in response to the abnormal deviation of said first derivative which results from said excess chlorine.

10. The process of claim 9, further comprising automatically adjusting said sodium hydroxide addition in additional response to the rate of chlorination of the total reactant solution.

References Cited bythe Examiner UNITED STATES PATENTS 2,950,178 8/60 Halfter et al. 23-253 3,051,631 8/62 Harbin et al 23-230 FOREIGN PATENTS 923,945 2/47 France. 

1. IN A PROCESS FOR THE PRODUCTION OF SODIUM HYPOCHLORITE FROM A SOLUTION OF CAUSTIC SODA AND A CURRENT OF GASEOUS CHLORINE IN A CHLORINATOR HAVING INLET AND DISCHARGE CONDUITS WHEREIN THE CONCENTRATIONS AND AMOUNTS OF THE MIXTURE OF SAID SODIUM HYPOCHLORITE, CAUSTIC SODA AND GASEOUS CHLORINE VARY WITH TIME AND SAID AMOUNTS OF CAUSTIC SODA AND GASEOUS CHLORINE ARE REGULATED TO MAINTAIN A SLIGHT ALKALINITY TO PREVENT DECOMPOSITION OF SAID SODIUM HYPOCHLORITE, SAID REGULATION BEING PERFORMED BY MEASUREMENTS OF THE REDOX POTENTIAL OF SAID MIXTURE, THE IMPROVEMENT COMPRISING MEASURING SAID REDOX PO-/ TENTIAL IN AT LEAST ONE OF SAID CHLORINATOR AND SAID ILET AND DISCHARGE CONDUITS, CONVERTING SAID REDOX POTENTIAL INTO AN IMPULSE CORRESPONDING TO THE FIRST DERIVATIVE OF SAID REDOX POTENTIAL RELATIVE TO TIME AND REGULATING BY SAID IMPULSE THE ADDITION OF AT LEAST ONE OF SAID CAUSTIC SODA AND CHLORINE, SAID ADDITION BEING MADE IN THE DIRECTION OF MAINTAINING SUBSTANTIALLY CONSTANT SAID FIRST DERIVATIVE OF SAID REDOX POTENTIAL. 